Potentiometric amplifier



ug- 3, 1948. K. RATH 2,445,399

POTENTIOMETRIC AMPLIFIER Filed sept. 28, 1945 2 Sheets-Sheet l Aug. 3, 1948. K, RATH 2,446,390

POTENTIOMETRIC AMPLIFIER Filed sept. 28, 1945 2 sheets-sheet 2 IN V EN TOR.

Patented Aug. 3, i948 POTENTIOMETRIC AMPLIFIER.

Karl Rath, New york, N. y., signor to naar Patents Corporation, New York, N, Y., a corporation of New York Application September-28, 1943, Serial No. 504,1@

My invention relates to potentiometric or follow-up type translating systems or ampliiiers for amplifying small input currents or potentials normally insuilcient for exciting the grid or other input element of a standard ampliner tube, such as the current or potential supplied by photovoltaic cells, thermo-electric elements or any other Weak current or potentialchanges btainable in response to any variable magnitude or condition to be indicated, recorded or otherwise utilized for operating an output or translatins device.

Known arrangements of this type make use of a balanced photoelectric translating system adapted to be unbalanced in accordance with the initial variations of the magnitude or condition to be indicated or translated, such as by'the aid of a sensitive galvanometer serving as a detecting and unbalancing means. The oii" balance or output current upon adequate amplification is utilized to counteract or compensate the initial variation, whereby to restore and maintain the balance of the system. As a result, the output current will be an exact replica of the initial current variations and may be utilized to operate a recorder or meter of substantially reduced sensitivity.

An object of the present invention is to pros cams. (ci. 17in-e5!) potentiometric translating arrangement embodythe basic principle thereof.

vide a system of the above character which is both simple in design as well as highly sensitive and stable in operation compared with poten tiometric balancing devices known according to the prior art.

Another object is to provide a system of this character wherein the mechanical parts and elements are reduced to a minimum, being in fact constituted solely by the galvanometer or other initial current responsive device excited by the weak input current or potential,

A further object is the provision of a device ofthe above character which can be easily balanced or adjusted to suit existing conditionsor requiremerlits substantially without expert knowledge or skil. f

These and further objects and novel aspects of my invention will become more apparent from the following detailed description taken in reference to the accompanying drawings forming part of this speciication and wherein:

Fig. 1 is a circuit diagram illustrating a simple ing the principles of the invention;

Fig. 2 is a graph illustrative oi the function and operation of Fig. l; and v i' Figs. .3, 4 and 5 are diagrams illustrating various modiiications of my invention embodying Like reference characters identify like parts throughout the different views of the drawings. With the aforementioned objects in view, the invention involves the use of a tuned or resonant electric circuit embodying means responsive to the variations of the initial magnitude or condition to be translated to vary the tuning or resonant frequency of said'circuit in relation to a fixed or predetermined reference frequency. The

varying relative departure of the tuning frequency with respect to said reference frequency is translated or converted into corresponding variations of output current of a translating vacuum tube or frequency detector of high re sponse sensitivity, The output current of said frequencyv detector or discriminator is then utilized by an inverse or negative feed-back arrangement to counteract the initialvariations or frequency modulation of said circuit in such a manner as to, instantly restore or maintain the balance between the resonant frequency of said circuit and said predetermined reference frequency. As a result, the'discriininating or output current which may have many times the in tensity of the original input currents, will vary substantially in exact synchron-ism with the latter and is utilized to operate a suitable output device, such as a meter, recorder or the like oi substantially reduced sensitivity compared with the original current responsive device such as a highly sensitive galvanometer, micro-ammeter or the like. f Y

Referring to Fig. 1, (I have shown a simple potentiometric amplifler'or translating arrangement embodying the principlev of the invention and utilizing a so-called space charge type discriminator or translating device disclosed in greater detail in U. S. Patent 2,208,091, to I.

.Zakarias, issued July 16, 1940, as a means for translating or converting the variations ofthe initial magnitude or condition into corresponding amplified output current changes. In the diagram shown, numeral i0 represents, by way of example, a thermo-electric element normally producing a current of insufficient potential for direct amplification by a standard vacuum tube amplifier. The element I is shown connected directly to the low resistance moving coil I I of a galvanometer type instrument in series with a small fixed compensating resistance I2 to be described presently. The galvanometer comprises the usual pole pieces I3 and I4 and magnet core I5 arranged to provide an air gap for the moving coil II. The terminals of the latter are connected through suitable flexible leads I6 and I1 commonly in the form of a pair of spiral balancing springs to the fixed terminal posts of the instrument.

'I'he moving coil II carries a pointer or other coupling 'element IB to which is Asecured the movable electrode 20 in the form of a thin metal vane or blade of a variable condenser element having a ,further fixed plate or electrode 2| suitably arranged for micrometric adjustment relative to the adjustable electrode 20. Thus, under normal conditions, the moving coll II excited by the current supplied by the thermocouple III will be deflected proportionally, resulting in corresponding variation of the capacc ity of the condenser element 2l, 2I.

This varying capacity is utilized in accordance with the invention to control or modulate the resonating frequency of aI tuned electric circuit and the variable tuning departure of said circuit with respect to a predetermined reference frequency is translated or converted into corresponding amplied current variations by the frequency detector or discriminator tube 22. The latter, in the example shown, comprises av heater 23, an equipotential cathode 2l, a rst or inner control grid 25, an accelerating or screen grid 20, a second or outer control grid 21 and an anode or plate 2l, all arranged substantially in the order mentioned. The electrode 2l of the translating condenser is connected to the .outer grid 21 and the cooperating electrode -2l is connected to the cathode by way of pointer Il, flexible lead I6 connected to said pointer in any suitable manner and resistance I2. The translating condenser 20-2I is shunted by an inductance coil 30 to form a parallel-tuned or antiresonant circuit connected between the grid 21 and the cathode 2l. The inner control grid 25 is shown excited by a source 3| of high frequency energy of substantially constant or invariable frequency supplied by an oscillator or high frequency generator of any suitable type known in the art, preferably a crystal controlled vacuum tube oscillator of known construction.

Item 32 represents the plate potential source shown in the form of a battery for simplicitys sake, the plate 2l being connected to the cathode through by-pass 'condenser Il for the high .frequency current. The accelerating or screen grid 2l is shown connected to the positive side of the source 33 through a voltage drop resistance 3S and is also by-passed to the cathode for high frequency currents by way of by-pass condenser 2l in a manner well known in the art. The circuit for the steady or direct plate current is completed through the indicating, recording or other output device 31 and the aforementioned compensating resistance I2 inserted in the input circuit of the thermocouple` and galvanometer. Condenser 38, besides completing the connection of the resonating circuit 20-2I-3l to the cathode, acts as a further by-pass condenser to prevent residual high frequency currents from 31.-

tering the input circuit. The output device 21 is shown by-passed by a compensating battery 21 in series with a resistance 39 to balance the constant. or steady output current through the meter or recorder.

In a frequency modulator-detector system of the type shown in Fig. 1, the direct output current io of the tube 22 varies in the manner indicated by the graph according to Fig. 2 as a function of thel relative departure between the exciting frequency impressed upon grid 25 and the natural or tuning frequency of the resonant circuit 20-2I--30 connected to the grid 21 and cathode 24. If the frequency of the resonant or discriminating circuit is equal to the operating or exciting frequency, corresponding to a capacity Co of the condenser element 20, 2l, then the direct or steady output current of the tube assumes a normal value Io equal to the plate current if the resonant circuit 20-2I-30 were omitted or the grid 21 disconnected entirely. If the resonant frequency of the circuit 2Ii2I3I increases beyond or decreases below the exciting frequency supplied by the source Il, or in other words, if the capacity of the condenser element 2li- 2i varies in either direction from its normal value Co as shown in Fig. 2, then the direct plate current in it will vary substantially linearly between upper and lower values I1 and In corresponding to the operating points Pi and Pa and capacity values C1 and Cz, respectively, of condenser 2li-2|.

By feeding back the steady or direct current of tube 22 to the compensating resistance I2 so as to counteract or substantially balance the initial potential supplied by the thermocouple III, and by proper design of the parameters and circuit constants of the system, an automatic balance or substantial compensation of the input voltage applied to the galvanometer may be effected and maintained in such a manner as to cause the output current through the translating device 31 having many times the value of the current supplied by the thermocouple I0 to follow the potential variations 'of the latter substantially instantly and in exact synchronism therewith.

The adjustment of the system according to a preferred embodiment is such that with a potential e of medium value supplied by the thermocouple I0, the output current of the discriminator or detector tube 22 has a value I0 as shown in Figure 2. Furthermore, the value R of the resistance I2 ls so designed that the voltage drop RIO caused by the output current substantially balances said input potential e. This balance condition e=ioR will be automatically maintained as the input potential changes, resulting in a variation of the output current io in synchronism with the input potential variations. In other words, any tendency of the galvanometer to deviate from its normal or balance position is instantly counteracted by a flow of oil-balance current in the input circuit resulting in a balancing potential drop or torque to return the moving coil l I to the balance position. The balancing operation is continuous and instantaneous to changes of input current or potential. All other circuit components subject to variation do not affect directly the operation of the system which is maintained at an inherently stable balance position with only the output current of the discriminator subject to change in producing the off-balance current flow.

If under the conditions outlined, the battery 31' and resistance 39 are designed to balance the mean output current In through' the device 21,

the latter may be oi the zero-center type for indicating or 'recording deviations of the input from an average or mean value. If ah indication or record between zero and maximum current is desired, battery 3l and resistance t9 should be adjusted to balance the current Iz through the device di, whereby the current through the output device 3l will vary between zero and manie mum inV synchronis'm with the` input voltage change, such as the potential supplied by the *other input units.

En order to avoid the use of a multi-stage direct coupled amplifier with its inherent defects and drawbacks due to drift, instability, etc. when higher output powers are required, the invention makes it possible to use an R. F. ampliiler oi high stability to secure a desired output current by means of the system described hereinbefore. An arrangement of this type is shown in Fig. 3, which otherwise diders from Fig. 1 by showing a photovoltaic, cell ll 'as an' input device. The latter, as is well known, supplies a potential unsuited for direct vampliilcation similar to the current and potential supplied by a thermo-electric device as shown in Fig. l. l have furthermore shown in Fig. 3 sa modiiied detecting condenser associated with the galvanometer and comprising a pointer tit carrying a blade or vane d2 of suitable conguration and arranged to cooperate with a iixed plate it mounted for micrometric adjustment indicated by an adjusting screw 'ttl'. By properly shaping the movable blade t2 any desired initial response characteristic may .be obtained, as is understood. l

l have furthermore shown in Fig. 3 an oscillator circuit associated with the detector tube tiltself, whereby to avoid a separate source or oscillater as shown in Fig. 1. For this purpose the cathode 2d, inner control grid 2b and screen grid 3b are utilized as a normal triodc having associated therewith. a, regenerative feed-back circuit to Iproduce continuous or sustained electrical oscillations. In the example shown, the resonant or tank circuit db has one end connected to the control grid 2b .through coupling condenser it and has its other end connected to the screen grid 2b through the coupling or by-pass condenser td. A suitable tap point of the inductance circuit db is connected to the cathode 2d, and grid 25 isbypassed to cathode through a grid leak resistance dl, whereby the system will function to generate sustained oscillations having a frequency determined by the resonant frequency of the circuit db' in a manner well understood. In order to insure proper function and stability of the system, the oscillator circuit and the resonant or discriminating circuit should be carefully shielded to avoid magnetic and other mutual coupling such as by the provision of grounded metal shields 3G' and te' well known to those skilled in the art.

In a discriminator of this type the output or platecurrent, in addition to the steady or direct current component as shown by Fig. 2, contains a high frequency component which in the case of Fig. 1 is liv-passed' to the cathode through condenser 3d. but in the embodiment of Fig. 3 is utilized for amplication by a stabilized high frequency amplifier and detector shown at bil.

As explained in detail in my Patent No. 2,248,197, f

- manner similar to the direct plate current of the tube it and is utilized to operate the output device bl and to provide the ofi. balance current for the potentiometric balance by returning itthrough the balancing resistance i2 in a manner `similar to Fig. l and understood from the above.l

Referringv to Fig. 4 l have shown a further modidcation of my invention. This'arrangement is similar 'to thataccording to Fig. l, with this difference that the exciting high frequency current is applied to the outer control grid 2l, while the resonating or discriminating circuit is shown connected between the inner control grid 2b and cathode it. Under these conditions, the system operates rin substantially the salme manner as in the case of Fig. l. with the only difference that the plate current change will be in a direction opposite to the change as shown in Fig. 2. This can be compensated lor by the design and choice of the winding sense of the galvanometer coil li.

Moreover, in the'embodiment of Fig. d, the resonating circuit is shown to consist of a xed condenser t@ and a variable inductance element controlled in accordance with the galvanometer deiiection. For this purpose, I have shown a split inductance coil havingelements b2 and bt and a metal vane or blade tb carried bythe pointer lb and arranged to move within the space between said inductance elements, whereby to vary the inductance thereof in a manner well known. This arrangement has the advantage that no electrical connection has to be madethrough the .pointer it to the metal vane 55 which preferably consists of copper or other highconducting material.

The design and adjustment of the system shown in Fig. ils such thatin the balance position the metal vane t5 which may be of circular shape as shown or of any other suitable conilguration, assumes a mean position within the space between the inductance elements b2 and bt to provide an inductance value resulting together with the fixed condenser 55 in aresonating frequency equal to the operatingor .exciting frequency supplied by the source 3l.A The potentiometric resistance i3 is againdesigned in such a manner as to produce a potential drop by the steady plate current Io to substantially balance a predetermined input potcntial supplied from a suitable device connected to .terminals 5l and exciting the galvanometer winding il' in a manner similar to that described in connection with Fig. 1. An initial deviation of the input-current or potential from 'an existing value in either direction will then result in an 'unbalance and subsequent instantaneous increase or decrease of the off balance output current to maintain the balance condition in a manner understood from the foregoing. By proper design and adjustment by the balancing battery or other source 3l', the output device 3l may be of the ordinary or zerocenter type in substantially the same manner as described hereinbefore.

In the preceding examples, the capacity or inductance varlationseil'ected by the initial detecting means such as a galvanometer or microammeter serves to modulate the lnatural or resonating frequency of a tuned circuit of a frequency detector or discriminator excited' by a source of substantially constant frequency, `whereby to derive an output current varying in proportion to the relative departure of said tuning frequency with respect to said constant exciting frequency. In the embodiment shown in Fig. 5, the varying reactance produced in accordance with the input potential or other condition to be translated, serves to frequency modulate a selfexcited high frequency oscillator, the .output of which is then detected by means of any known frequency discriminator to produce an amplified direct current varying exactly as the initial input changes and serving to operate' both the output device and to provide the off balance or negative feed-back current for the input circuit necessary to maintain the balance condition of the system.

For this purpose, I have shown in Fig. a multielectrode Vacuum tube 60 of the penta-grid converter type comprising in a known manner, a heater 6|, an equi-potential cathode 62, a rst control grid 63, an accelerating or anode grid 64, a screen grid 65 enveloping said accelerating grid, a suppressor grid 66 internally connected to the cathode in a known manner and a plate 61, all arranged substantially in the order mentioned'.

A tuned resonant or tank circuit comprising an inductance coil 1U shunted -by a variable condenser having electrodes 12 and 13` is connected vacuum tube 1I shown inthe example illustrated and comprising a heater 19, a cathode 6U, a control grid 6I, an amplifier plate or anode l2 and a rectifier plate 63 adjacent to the cathode I0. The high frequency output currents are applied from the circuit 16 by way of coupling coil 11 to the diode plate 63 and to the cathode 60 through a diode load resistance Il by-passed for high frequency current by a condenser ln a manner well known. A portion of the rectified current is applied from the diode resistor 64 to a coupling resistor 65 by way of a coupling condenser and a suitable portion of the rectied potential developed by the coupling resistor 66- is shown applied to the control grid 8| for further ampliication by the triode section of the tube.v Item 86 represents a biasing network comprising a condenser-shunted resistance in the cathode return lead of the tube to provide suitable steady operating bias for the control grid 8| in a manner well known. The amplified output current of the tube 16 is applied to the output device 31 and passed through the potentiometric balance resistance l2 in substantially the same manner as shown and understood from the preceding exempliflcations of the invention.

Arrangements of the type described, besides being simple from a mechanical standpoint in requiring a minimum of mechanically moving parts and adjustments, have the advantage of extreme sensitivity by the utilization of the resonance effect affected by the initial input variations and resulting in increased stability and operating safety as well as low manufacturing and maintenance cost of instruments of this type.

The operating frequency used in arrangements aforedescribed may be chosen to suit any existing and special requirements. Practical con- 62, whereby to provide a self-oscillating regenerative circuit capable of producing sustained oscillations having a frequency determined by the capacity of the condenser 12, 13. The oscillations are transmitted to the output or plate circuit 16 by electron coupling in a manner well known and understood by those skilled in the art.

The variable condenser in the example shown comprises a xed plate orA electrode 12 and a movable electrode blade 13 carried by the torsion string or wire of a normal string galvanometer having an exciting coil (not shown). In other words, blade 13 takes the place of the usual mirror provided in galvanometers of this type.

In an arrangement of this type. the varying input current supplied by the source 51 tends to deflect the galvanometer electrode 13 to approach or recede from the fixed electrode 12, whereby to vary the electrical capacity between said electrodes in a manner well understood. As a result, the frequency of the oscillations generated tends to change in the one or other direction from a center or normal frequency. In order to detect the frequency changes any known type of frequency detector or discriminator may be connected to the output circuit. In the example shown the discriminator comprises the resonant siderations have .shown that a frequency between about l to 1U mc. and higher results in an eilicient and mechanically simple construction, since the size and weight of the adjusting element (2l, l2, 55 and 13) decreases as the operating frequency is increased. As an example, when using a variable condenser of about one to :lecci and a frequencyof about 5 mc., the adjusting element will be of such small size and weight as not to impair the accuracy and sensitivity of galvanometer of known construction.

The oscillator or operating :frequency used should be maintained constant to insure accuracy and reliability of the indication or record. When lower frequencies are used, the oscillator may be stabilized by the aid of a piezo-electric crystal or equivalent device, or any other well known means of frequency stabilization may be employed for the purpose of the invention. If a double-grid electron tube is used as a frequency detector as shown in Figures 1, 3 and 4 which as quency, the grid potentials in a detector tube of this type will be in quadrature or at a relative phase difference, resulting in no effect on the direct or steady plate current of the tube (see current In according to Figure 2) If, under this is necessary to rebalance the system .is toi-esdniet the variable' condenser or induction con cy operating the micrometric adjustment and/or to i'eadjust the operating frequency, until the steady plate current again remainsthe same in both the on or od-positicn oi the resonant circuit.

as is understood, condenser d-l in Figure i and condenser tid-dt in Figure 3. may be provided with two stationary plates arranged on op posits sides of the movable electrode .to obtain an increased initial capacity if desirable.

it will be evident from the foregoing that the accesso age and varying in sense and magnitude in proportion to the relative frequency departure between said source and the resonant frequency of l said impedance means, and further meansv for combiningsaid secondary high frequency voltage with saidsourc voltage to produce a direct output current of amplitudevarying in sense and invention is not limited to the specific details,

arrangement of parts and constructions shown and disclosed herein for illustration, but that'the s underlying thought and basic principle are susceptible of numerous variations and modications coming within the 'broader scope and spirit of my invention as deined in' theappended claims. The specification and drawings are accordingly to be regarded in an illustrative rather than in a limiting sense.

l claire:

l.' in electrical system comprising an input device supplying a relatively weak direct current input voltage; a frequency converter comprising aresonantimpedance means. a source of auxillary high frequency voltage' having a frequency normally relatively equal to the resonant frequency of said impedance means, coupling means between saidl resonant impedance means and said source to develop a secondary high frequency voltage by said impedance means having a phase normally at Q with respect to the phase of said source voltage and varying in sense and magnitude in proportion to the relative frequencydeparture between said. source and the resonant frequency of said impedance means, and further means for combining said secondary high :fre-

quency voltage with said source voltage to produce a direct output current having an amplitude vagin sense and magnitude in proportion to said'frequency departure; a current responsive device connected to said input device and means operativelyassociated therewith to edect a relative frequency departure between said source and the resonant frequency of said impedance means; a resistance traversed by'said direct output curv rent and connected in serieswith said input device and said current responsive device to produce a potential drop substantially counterbalancing said input voltage, whereby to restore the frequency balance between said source and the resonant frequency oi' said impedance means and to maintain a condition of electrical equilibrium between said direct current input voltage and said output current; and translating means responsive to said output current.

d. dn electrical system comprising an input device supplying a relatively weak direct current magnitude in proportion to said frequency departure; a galvanometer connected to said input device and having a movable element adapted to control the resonant frequency of said impedtially counterbalancing said input voltage, where' by to restore the balance between the frequency ot said source and the resonant frequency of said impedance means and to maintain a condition of electrical equilibrium lbetween said input voltage and said output current; and translating means responsiveto said output current. i

3. In an electrical system comprising anl input device supplying a relatively weakI direct current input voltage; a frequency converter` comprising a resonant circuit including an induction coil and a variable condenser having a uned electrode and movable electrode, a source of auxiliary high frequency voltagehaving a constant frequency normally equal to the tuning frequency of said circuit, coupling means between said circuit and saldsourceto develop a secondary high irequency voltage by said circuit having a phase normally at a 90 angle with respect 'to said source voltage and varying in sense and magnitude in proportion to the relative frequency departure between said source and the tuning frequency of said circuit, and further means for combining said secondaryv high frequency voltege with said source voltage, to produce a direct output current of amplitude varying in sense and magnitude in proportion to said frequency departure; a galvanometer connected to said input device and having a movable element actuating said movable electrode, whereby to vary the tuning frequency ofsaid circuit and to eect a relative frequency ldeparture therebetween and said source; a resistance traversedy 'by said output current and connected in series with said input device and said galvanometer, to produce a potential drop substantially counterbalancing said input voltage, whereby to restore the frequency balance between said source and the resonant frequency of said circuit and to maintain a condition of electrical equilibrium between said input voltage and said output current; and translating means responsive to said output current.

4. An' electrical system comprising van input device supplying a relatively weak direct cur` input voltage; a frequency converter comprising a resonant impedance means, a source of auxil.

lary high frequency voltage having a constant frequency normally equal to the resonant frequency of said impedance means, coupling means between said resonant impedance means and said source to develop a secondary high frequency voltage by said impedance means having a phase normally at with respect to said source voltrent input voltage; a` frequency converter comprising a resonant circuit including a condenser and an induction coil, a source of auxiliary high frequency voltage having a constant frequency normally equal to the tuning frequency of said circuit. coupling means between said circuit by said source .to develop a secondary high frequency voltage by said circuit having a phase normally at a 90 angle with respect to said source voltage and varying in sense and magnitude in proportion tothe relative frequency departure between said'v source andthe tuning frequency of said circuit, and further means for combining said secondary high frequency voltll Y age with said source voltage to produce a direct output current of amplitude varying in sense iand magnitude in proportion to said frequency departure; a galvanometer connected to said input device and having a movable element carrying a metallic mem'ber arranged to cooperate with said induction coil to vary the inductance thereof, to thereby effect a relative frequency departure between said/source and the tuning frequency of said circuit; a resistance traversed by said output current and connected in series with said input device and said galvanometer to produce a potential drop substantially counterbalancing said input voltage, whereby to restore the fr quency balance lbetween said source and the ng frequency of said circuit and to maintain a condition of electrical equilibrium between said input voltage and said output current; and translating means responsive to said output current.

5. An electrical system comprising an input device supplying a relatively weak direct curl rent input voltage; a frequency converter comprising an electronic tube having at least a cathode, an anode, a pair of control grids and a screen grid interposed between said control grids, a resonant impedance means connected between said cathode and one of .said control grids, a source of auxiliary high frequency voltage having a frequency normally relatively equal to the resonant frequency of said impedance means and connected between the other of said control grids and said cathode, said source being substantially exteriorly decoupled from said impedance means, to produce a direct anode current having an amplitude varying in sense and magnitude in proportion to the relative `frequency departure between said source and the resonant frequency of said impedance means; a current responsive device connected to said input device and having means operatively associated therewith to effect a relative frequency departure between said source andthe resonant frequency of said impedance means; means including a balancing resistance in series with said input device and said current responsive device to produce a voltage drop varying in proportion to said direct output current and substantially counterbalancing said input voltage, whereby to restore frequency balance between said source and said resonant impedance means and to maintain a condition of electrical equilibrium between said input voltage and said output current; and translating means responsive to said output current.

6. An electrical system comprising an input device supplying a relatively weak direct current input voltage; a frequency converter comprising an electronic tube having at least a cathode, an anode, a pair of control grids and a screen grid interposed between said control grids, a resonant impedance means connected between said cathode and one of said control grids, a source of auxiliary high frequency voltage of substantially constant frequency normally equal to the resonant frequency of said impedance means and connected to the other of said control grids and said cathode, said source being substantially exteriorly decoupled from said impedance means, to produce a direct anode current having an amplitude varying in sense and magnitude in proportion to the relative frequency departure between said so u rce and the resonant frequency of said impedance means; Aa galvanometer connected to said input deviceand having movable means adapted to control the resonant frequency of said impedance means; means including a balancing resistance in series with said input device and said galvanoxneter, to produce avoltage drop varying in proportion to said direct output current and substantially counterbalancing said input voltage, whereby to restore the frequency balance between said source and said resonant impedance means and to maintain a condition of electrical equilibrium between said input voltage and said output current; and translating means responsive to said output current.

7. An electrical system comprising an input device supplying a relatively weak direct current input voltage; a frequency converter comprising an electronic tube having at least a cathode, an anode, a pair of control grids and a screen grid interposed between said control grids, a resonant circuit comprising an induction coil and a variable condenser having a fixed electrode and a equal to the tuning frequency of said circuit and connected to the other of said control grids and said cathode, said source Ibeing substantially exteriorly decoupled from said resonant circuit, to produce a direct anode current having an amplitude varying in sense and magnitude in proportion to the relative frequency departure between said source and the tuning frequency of said circuit; a galvanometer connected to said input device and having a movable element actuating said movable electrode, whereby to vary the resonant frequency of said circuit and to produce a relative frequency departure between said source and the tuning frequency of said circuit; means including a balancing resistance in series with said input device and said galvanometer and adapted to produce a voltage drop varying in proportion to said direct output current andsu'bstantially counterbalancing said input voltage, whereby to restore the frequency balance between said source and the tuning frequency of said circuit and to maintain a condition of electrical equilibrium between said input voltage and said output current; and translating means responsive to said output current.

8. An electrical system comprising an input device supplying a relatively weak direct current input voltage; a frequency converter comprising an electronictube Ahaving at least a cathode, an anode, a pair of control grids, and a screen grid interposed between said control grids, a resonant circuit including a condenser and an induction coil and connected to one of said control grids and said cathode, a source of auxiliary high frequency voltage of constant frequency normally equal to the tuning frequency of said circuit and connected between the other of said control grids and said cathode, said source being substantially exteriorly decoupled from said circuit, to produce a direct' anode current having an amplitude varying in sense and magnitude in proportion to the relative frequency departure between said .source and the tuning frequency of said circuit;

a galvanometer connected to said input device and having a movable element carrying a metallic element actuated thereby and cooperating with said induction coil to control the inductance thereof to thereby effect a relative frequency departure between said source and the tuning frequency of said circuit; means including a balancing resistance in series with said input device and said galvanometer for producing a voltage REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Name Date Gilbert Aug. 13, 1935 Number Number 14 Name Date Temple June 15, 1937 Gilbert Nov. 15, 1938 MacLaren, Jr Apr. 11, 1939 Rosene July 11, 1939 Vance --Feb. 20, 1940 Zakarias July 16, 1940 |Parker et al. Oct. 15, 1940 Cohen ....vJan. '7, 1941 Zakarias .1 May 13, 1941 Thompson Nov. 4, 1941 Bach Jan. 20, 1942 Weymouth Apr. 21, 1942 MacKay May 26, 1942 Lamb June 9, 1942 Eberhardt et al. Sept. 29, 1942 

